Stentor coeruleus, a unicellular hetrotrich,possesses both light intensity-and color (wavelength)-sensory "visions", representing the first clearly identifiable "visual sensory system" based on stentorin as the photoreceptor,as an alternative to the well established rhodospin-based visual sensory system in higher animals. This aneural organism is also able to perceive the direction of light propagation. Light sensing is manifested in the form of ciliary beat reversal by the organism. The photosensory process in this cell can be described schematically as follows: Light signal to Absorption by photoreceptor to Signal generation and amplification to Response (ciliary stroke reversal) We propose to study the possible involvement of a light-induced proton release (ph change) from the photoreceptor as a primary mechanism for the generation of a light signal. The initial signal in the form of proton release from the photoreceptor complexes has been suggested to trigger subsequent transduction steps that include calcium ion influx from the extracellular medium,thus amplifying the initial light signal. It is proposed that the calcium ion influx causes stentor cell to reverse the ciliary beat direction and subsequently steer away from the harmful light source. Specifically, we plan to elucidate the mechanisms of the key sensory transduction steps. The photoreceptor stentorin will be isolated and further characterized,and its primary photoprocesses in solution and in the photoreceptor organelle (pigment granule) as well as in the intact cell will be investigated. The role of proton release from the excited state of stentorin as an initial sensory signal (a ph change-evoked receptor potential) will be illustrated by using ph probes in the pigment granule-entrapped liposomes and in vivo. We propose that a transient intracellular ph drop resulting from the proton release from pigment granules serves as an electrical signal to open calcium in the cell/ciliary membranes. The influx of calcium ions then trigger a ciliary beat reversal. This hypothesis will be further tested in the present study.